Development of high-luminance ultraviolet light emitting elements is recently progressing toward practical use. Light emitting elements with the emission wavelength of the order of 300 nm have been proposed using various materials such as gallium nitride and solid solution thereof. For the changeover to the shorter wavelength of the emitting wavelengths of these solid-state light emitting elements, there is a large demand in fields such as high densification of recording media and others. To date, as the candidates for far ultraviolet light emitting element on a wavelength of the order of 200 nm, diamond, cubic boron nitride crystal (hereinafter, denoted by cBN) and aluminum nitride have been proposed and studies for application thereof are in progress.
In searching for materials for a high-luminance light emitting element in the far ultraviolet region, important characteristics include: having broad band gaps and chemical stability, and preferably to be direct transition type semiconductors, and the like. Except for above described materials, the solid-state light emitting materials with far ultraviolet light emission characteristics of the order of 200 nm include hexagonal boron nitride crystal (hereinafter, denoted by hBN) having about 5.8 eV band gap and being a direct transition type semiconductor. But there have been factors to prevent its realization. hBN has been used for a long time as a chemically stable insulator material, is synthesized by gas phase reaction of boron oxide and ammonia, and is now utilized in many forms (such as powder, sintered body, and film form).
However, hBN obtained by the above described gas phase reaction has contained impurities to make it difficult to obtain hBN having the far ultraviolet light emission characteristics corresponding to its specific band gap. In order to use this material as the high-luminance light emitting element in far ultraviolet region, it is necessary first to establish methods to synthesize highly pure single crystals, but there has been no report until now that the highly pure hBN single crystal with expected light emission characteristics has been successfully obtained by a hBN synthesizing method, aiming at its potential ability as a solid-state far ultraviolet light emitting element with the emission wavelength of the order of 200 nm.
As for the synthesizing method, hBN has been known to be synthesized by the thermal decomposition reaction or by the gas phase reaction between boron compounds such as boron oxide and ammonia, but it has been difficult to obtain highly pure single crystals by these reactions. Especially, they have never been considered established as the manufacturing methods of single crystal materials to use for semiconductors or the like.
On the other hand, cubic boron nitride crystal, a high-pressure phase of hBN, has been known to be synthesized by using hBN or the like as the raw material and boronitride of alkali metal or alkali earth metal as a solvent and by recrystallizing said raw material in said solvent under high-temperature and high-pressure of 55,000 atmospheric pressure and 1,600° C. Obtained cBN single crystal has high hardness next to the diamond, and is widely used as a super hard material, and this procedure for synthesizing cBN has already been established industrially.
Because cBN synthesized in this way also has a broad band gap (e.g. 6.3 eV), it has been studied for a long time as a solid-state short wavelength light emitting element. However, every cBN single crystal hitherto reported is colored in amber, orange or the like, and the light emitting behavior corresponding to cBN specific band gap has not yet been able to be observed in this situation. As a possible cause thereof, large effect of impurities contained in the cBN crystal may be nominated. Therefore, in order to use the cBN single crystal as a material having specific light emission characteristics corresponding to the band gap of said crystal, establishment of synthesis reaction to achieve higher purification of cBN single crystal has become an important subject to study, as well as full understandings of light emission characteristics specific to cBN.
Under such background, it has been reported that synthesis of hBN single crystal was tried under the condition of cBN synthesis daringly changing the temperature and pressure conditions to those at which hBN is produced stably (non-patent literature 1). However, from the crystal-growth solvent used in the synthesis experiment in this report, only colored cBN crystals were obtained, and about the hBN crystal that was formed concurrently as a by-product, there was no description on the light emission behavior thereof at all or no suggestion on short wavelength light emission thereof.
In such a situation, the inventors of the present invention intensively studied the synthetic conditions for obtaining highly pure cBN single crystals. Consequently, they found factors necessary to obtain highly pure cBN single crystals, and thus succeeded in synthesizing highly pure cBN single crystals having optical characteristics specific to the cBN crystal, and reported in an academic literature (non-patent literature 2). This synthetic procedure was, in short, after establishing a clean and dry nitrogen-atmosphere, crystals were grown using highly pure solvent (such as barium boronitride) purified with utmost care. By this procedure, highly pure cBN single crystals were successfully obtained (non-patent literature 2).    Non-patent literature 1; H. Akamaru, A. Onodera, T. Endo, O. Mishima, J. Phys. Chem. Solids, 63, 887 (2002).    Non-patent literature 2; T. Taniguchi, S. Yamaoka, J. Cryst. Growth, 222, 549 (2001).